An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths in that plane to a point or small area of convergence near the tube screen. In one type of inline electron gun, such as that shown in U.S. Pat. No. 3,873,879, issued to R. H. Hughes on Mar. 25, 1975, the main electrostatic focusing lenses for focusing the electron beams are formed between two electrodes referred to as the first and second accelerating and focusing electrodes.
The concept of utilizing two electrostatic focusing lenses to form an effective larger main focus lens is disclosed in U.S. Pat. No. 2,975,315 issued to C. S. Szegho on Mar. 14, 1961, in U.S. Pat. No. 3,852,637 issued to E. Yamazaki et al. on Dec. 3, 1974, and in U.S. Pat. No. 4,334,169 issued to S. Takenaka et al. on June 8, 1982. In each of these patents, four electrodes are used to form the two electrostatic focusing lenses. In each patent, one lens is formed by three of the electrodes with the center electrode being excited with a lower voltage than the two-side electrodes which are electrically connected. The other lens in these patents is formed by two electrodes excited with different voltages.
An inline electron gun wherein a bipotential electrostatic focusing lens is expanded in size is disclosed in U.S. Pat. No. 4,370,592 issued to R. H. Hughes et al. on Jan. 25, 1983. In this patent, the enlarged lens is formed by setting back or recessing the three inline apertures in each of two focus electrodes so that the rims around the recesses which face each other provide the primary control in forming the main focus lens.
The concept of forming an astigmatic lens in the beam forming region of an electron gun by the inclusion of a slot in the first electrode grid is disclosed in the following patents: U.S. Pat. No. 4,242,613 issued to J. Brambring et al. on Dec. 30, 1980; U.S. Pat. No. 4,251,747 issued to G. A. Burdick on Feb. 17, 1981; and U.S. Pat. No. 4,272,700 issued to F. K. Collins on June 9, 1981. Slots in the second electrode grid are disclosed in the following patents: U.S. Pat. No. 3,497,763 issued to J. Hasker on Feb. 24, 1970; U.S. Pat. No. 3,866,081 issued to J. Hasker et al. on Feb. 11, 1975; and U.S. Pat. No. 4,234,814 issued to H. Y. Chen at al. on Nov. 18, 1980.
The foregoing patents provide varying contributions to the cathode-ray tube art, which in themselves are valuable, but the patents do not suggest how the varying concepts disclosed therein can be combined to obtain an electron gun having decidedly improved performance.